In subsea cable installations, the cable insulation must be protected from humidity that may lead to electrical breakdown. For example, in cable insulation made from for example polymers, or oil impregnated paper, there are possibilities for water treeing when the humidity is above a certain level that after some time may lead to electrical breakdowns. Different types of water barriers are used in different cable constructions, but the dominating material used for water barriers is lead. This is particularly the case for high voltage power cables. Lead has proved to be a reliable and sturdy sheathing material in this connection, but it has some well known drawbacks. Particularly with large high voltage underwater cables, the weight issue is important because lead contributes much to the total weight. Heavy cables add cost in the whole value chain; in production, under transport, storage and deployment. Even when the cable reaches its economic lifetime, discarding a heavy cable adds cost. Deploying underwater cables in deep waters is of particular interest because the strain in the cable can be considerable.
One other problem with lead is when cables are used dynamically, i.e. when the cable is not used in a fixed position. Lead sheath has a low fatigue limit, making it unsuitable for dynamic applications. Besides, lead is considered as non-environmentally friendly and for some markets replacement of lead is required.
For long high voltage underwater cables, capacitive charges and resulting currents will initiate problems if not dealt with. Preferably these currents are led radially out into the sea water and not led longitudinally along the cable. This is accomplished by using non insulating layers instead of insulating layers in the cables. One alternative, not very convenient in subsea cables, is to have controlled discharging of capacitance at regular intervals. The added cost of using discontinuous deployed cables makes this alternative a prohibitive solution.
Among other metallic materials used as water barriers, aluminum is most often applied, for instance as a corrugated tube protection. The density of aluminum is 2.7 kg/dm3 while lead is 11.4 kg/dm3. However, such corrugated aluminum barriers are normally used on and cables, not on subsea cables. Subsea cables have different and often stricter requirements.
One advantage with lead sheathing in cables is its plasticity properties. The industry has for many years applied lead onto cables with a thickness of for example 5 mm. Such cables are still straight forward deployable.
In GB2105486, a fiber optic cable is foreseen with a water barrier in the form for instance of an aluminum plastics laminate. In fiber optic cables, the problem of water treeing is not present. In the art, laminates intended for use in medium and high voltage cables, comprising a metal water barrier laminated with layers comprising semi conductive polymer layers to discharge capacitive charges to ground (here: sea water) remains to be presented.
WO0146965 describes a water swellable tape internal to a metallic screen, immediately beneath the latter. In the same document another embodiment is described having a thin aluminum foil arranged and glued beneath an outer polymeric sheath, the aluminum foil acting as a barrier to water penetration.